Gulp watch: how to run in terminal?

Question

gulp.js

Hello. Tell me what command to run gulp watch in the terminal (when driving in gulp watch, it writes an error)?
Here is the content of gulpfile.js

const gulp = require('gulp');
const sass = require('gulp-sass')(require('sass'));
const sourcemaps = require('gulp-sourcemaps');
const watch = require('gulp-watch');

function buildStyles() {
    return gulp.src('./scss/**/*.scss')
        .pipe(sourcemaps.init())
        .pipe(sass().on('error', sass.logError))
        .pipe(sourcemaps.write('./'))
        .pipe(gulp.dest('./css'));
};

exports.buildStyles = buildStyles;
exports.watch = function () {
    gulp.watch('./scss/**/*.scss', ['scss']);
};

Compilation of scss is launched via the gulp buildStyles command, but I can't run gulp watch (.